Manufacturers of electrical equipment and appliances, such as those designed to operate at 110 VAC or 220 VAC, have customarily incorporated ground fault interrupt (GFI) circuitry in the supply path from the power supply input derived from a power outlet, such as a wall outlet, and the electrically powered device or load proper, as a safety measure to protect the user. A typical example of a power supply interconnect network configured for this purpose is diagrammatically illustrated in FIG. 1 as comprising an upstream power transformer 10 from which a 220 VAC line is coupled to a circuit breaker panel 12. A respective leg of the power supply line proper includes a line or ‘hot’ lead 21 and a return or neutral lead 22. From the circuit breaker panel 12 the leads are typically fed to and terminate at a female electrical receptacle that is mounted at a wall outlet 30, and may include a connection from earth ground to the neutral lead. The electrical appliance or equipment 40 that is to be powered from the wall outlet contains a GFI circuit 50 as part of the power feed that is used to connect the appliance to the wall outlet receptacle. GFI circuit 50 typically contains a sense transformer 51 containing a pair of windings derived from the line voltage lead and the return or neutral lead which are used to deliver power through a GFI-controlled switch 55 to an electrically powered device or load 60.
The GFI circuit 50 operates as follows. As long as the equipment is connected as shown in FIG. 1 and there is no inadvertent contact (as by way of an equipment user) between the power supply leads and ground, then whatever current is drawn by the load by way of the line voltage lead 21 will be returned to the source (the line power transformer) via the neutral lead 22. As a result, the magnetic fields produced by the respective line voltage lead and neutral lead windings in the sense transformer 51 will balance one another, so that there will be no resulting differential magnetic field of sufficient strength to open the contacts 56 and 57 of GFI switch 55. However, should there be an inadvertent contact at the electrical device between one of the power supply leads and ground, then the magnetic fields produced by the respective line voltage lead and neutral lead windings in the sense transformer 51 will be out of balance, yielding a net magnetic field of sufficient strength to cause the GFI switch 55 to open its contacts 56 and 57, thereby interrupting the connection from the power supply and the electrical device and ostensibly protecting the user.
Now although GFI circuits of the type shown in FIG. 1 have worked reasonably well, they have recently begun to suffer faulty operation owing to the incorporation of additional protection circuitry that is now becoming commonplace in modern electronic equipment. Specifically, as shown in FIG. 2, it is now common practice to incorporate electromagnetic interference (EMI) filter capacitors as part of the electronic equipment, in order to control the emission of high frequency electrical (digital) noise, which is generated by the internal circuitry of the equipment. As shown in FIG. 2, the EMI protection customarily consists of a pair of high frequency by-pass capacitors C1 and C2, which are respectively connected between the line voltage lead 21 and the neutral lead 22 and an earth ground lead 23, so that they provide a current path for high frequency noise from the line or neutral wires to earth ground. However, as a result of these by-pass capacitors, some of the return current now flows through the earth ground lead, so that there is an imbalance in the currents flowing through the respective windings of the sense transformer 51 of the GFI switch 55. This leads to a ‘false’ triggering of the GFI switch, as the electrical device has been designed to include EMI filtering, which is working correctly. This inability of the GFI circuit to discriminate between designed-in current paths to ground (such as EMI filters which may produce a current imbalance between the line and neutral leads) and a true external ground fault, which may be hazardous to the user, constitutes a problem for users of GFI-protected equipment.